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Glass Mountain obsidian flow, Medicine Lake volcano

Glass Mountain is a spectacular, nearly treeless, steep-sided rhyolite and dacite obsidian flow that erupted just outside the eastern caldera rim and flowed down the steep eastern flank of Medicine Lake volcano. Thirteen vents along a northwest-southeast trending fissure were active during this eruption, which occurred about 950 years ago. Ten northwestern vents and the southeastern-most vent produced domes, while lavas of the intervening three vents coalesced to form the Glass Mountain flow, one cubic kilometer (0.25 cubic mile) in volume. The flow consists of three dacitic eastern lobes which grade westward to rhyolite and are overlain by rhyolite lobes. Glass Mountain first erupted dacite containing mafic magmatic inclusions and last erupted rhyolite without inclusions. Granitic inclusions are rare in Glass Mountain, but were probably derived from underlying rock related to Medicine Lake volcano itself.

The precise age of Glass Mountain and its preceding pumice deposits has been a matter of discussion for some time. A radiocarbon age of 885+/- 40 years B.P. was obtained on a dead cedar tree without limbs or bark that is preserved in the edge of one of the distal tongues of the flow. The dated material consisted of a piece of exterior wood containing about 30 annual growth rings. This age may be too old, because some of the outside of the tree is missing. The tephra deposits that precede the flow and domes may be somewhat older but are constrained to be less than about 1050 years B.P. by the Little Glass Mountain and Lassen data.